The present invention relates to a diagnosis system for internal combustion engines, and more particularly to a system for diagnosing failure of two or more devices in an engine mounted on a motor vehicle.
An electronic fuel-injection system of the type which is provided with a computer operable to compute various factors, such as throttle position, ignition signal, intake manifold vacuum and coolant temperature in order to produce output signals for driving solenoid-operated fuel injection valves is known. The solenoid operated fuel-injection valves are driven in dependency on the ignition signal which represents the position of the ignited cylinder and the ignition timing. A conventional diagnosis system is provided with a first diagnosis system for a control signal generating system such as the ignition signal generating system and a second diagnosis system for a driven system such as the fuel injection system. Both systems are separately provided, and each diagnosis system has an indication device so as to signal an alarm in the event of a fault in its own system regardless of the conditions of the other system. In such a system, if a failure occurs in the control signal generating system, the driven system is not operated. Therefore, the first diagnosis system indicates the fault in the control signal generating system and the second diagnosis system also provides the alarm in spite of a normal condition of the driven system as if there is a fault in the system.